In long-distance and high-speed optical transmission systems, optical modulators (hereinafter referred to as “LN modulator”) utilizing the electro-optic effects of a dielectric such as LiNbO3 are used. Coherent light is input to the LN modulator from the outside, and the LN modulator modulates the light by changing the optical phase (hereinafter simply referred to as “phase”) with a high-speed electric signal. When the light is modulated, a DC bias is applied to the optical modulator to create a phase reference. The LN modulator has a bias shift phenomenon in which the phase reference point (for example, the optimum value of the bias voltage) changes with time. This phenomenon is called “DC drift”. Automatic bias control (ABC) is performed to follow the DC drift. The ABC has also been employed in optical digital coherent communication, which is the main stream of today.
In an I/Q modulator that handles signal components whose phases are offset by 90° from each other, bias control is individually performed at each of the I (in-phase) channel, the Q (quadrature) channel, and the phase shifter that adjusts orthogonality between I and Q. When bias voltage control of the I channel, the Q channel, and the phase shifter is performed in time division, in general, while the bias voltage control of one branch (for example, I channel) is being performed, the bias voltages of the other two branches are fixed.
A low frequency pilot signal is generally used for drift control of the bias voltage. The pilot signal is superimposed on the bias voltage applied to the optical modulator, and the phase of the pilot signal included in the output light of the modulator is detected to control the bias voltage to the optimum operating point. FIG. 1 is a schematic diagram of a superimposed pilot signal and an observation pilot signal in bias voltage control. The pilot signal to be superimposed is indicated as one period of a sinusoidal waveform with frequency f0 [Hz]. The waveform of the pilot signal to be observed varies depending on the deviation direction of the applied bias voltage (inclination of light power). At the bias voltage A where the inclination of the light power characteristics is negative, a signal whose phase is inverted with the same frequency f0 as the superimposed pilot signal is observed. At the bias voltage B where the inclination of the light power characteristics is zero, a signal with twice the frequency (2f0) is detected and the superimposed pilot signal is not observed. At the bias voltage C where the inclination of the light power characteristics is positive, a signal having the same frequency f0 and the same phase as the superimposed pilot signal is observed. The control direction (sweep direction) of the bias voltage is determined from the frequency and the phase of the observed pilot signal. The bias voltage is swept to the position where the inclination of the light power characteristics curve is zero (a valley or a mountain of the light power characteristics curve).
A method of determining the initial value of the bias voltage to be applied to each of the two modulators in a short time has been offered.
The light power characteristics of FIG. 1 are indicated as an ideal cosine curve. However, the output light power characteristics of the LN modulator may deviate greatly from the ideal cosine curve depending on the bias voltage values of the I channel, the Q channel, and the phase shifter and the signal amplitude at the time of high speed data signal communication.
FIGS. 2A-2C are diagrams illustrating the distortion of the light power characteristics with respect to the bias voltage (hereinafter simply referred to as “light power characteristics”). In these figures, the light power characteristics at the time of the bias voltage control of the I channel in the ABC process at the time of start-up without the input of the data signal (detection of the emission/quenching reference phases of I channel and Q channel and setting of the quadrature phase between I and Q) are plotted. The bias voltage of the I channel is normalized to emit light at 0 V and quench light at 1 V (Vπ=1 V). FIG. 2A depicts an ideal shape. At this time, the bias voltage of the Q channel is 1 V and the bias voltage (φ) of the phase shifter is 1 V. Distortion is seen in FIG. 2B and FIG. 2C. In FIG. 2B, the bias voltage of the Q channel is 0.8 V, and the bias voltage (φ) of the phase shifter is 1 V. In FIG. 2C, the bias voltage of the Q channel is 0.6 V and the bias voltage (φ) of the phase shifter is 1 V. In FIG. 2B and FIG. 2C, the bias voltage at which the light power is minimized deviates from the point of 1 V, and the light power is bilaterally asymmetric with respect to the bias voltage at which the light power is minimized. Such distortion in the phase direction means that emission/quenching of the LN modulator is not performed normally.
The shape of the light power characteristics curve is changed each time respective bias voltages of the I channel, the Q channel, and the phase shifter are swept. The distortion of the light power characteristics is not necessarily reduced by the sweep of the bias voltage. The distortion may remain until the end while changing the shape of the curve.
The following is a reference document.
[Document 1] Japanese Laid-open Patent Publication No. 2015-114499.